Currently, a thin film transistor is comprised of source and drain electrodes to supply electrons to a semiconductor layer or to bring the electrons out of the semiconductor layer, and a gate electrode overlapped with the semiconductor layer. An insulating layer is disposed between the gate electrode and the semiconductor layer to thereby control a current. The thin film transistor may be used as an integrated driving circuit, as a switching device to independently drive each sub-pixel of a liquid crystal display (LCD), or as a switching and driving device to independently drive each sub-pixel of an organic light-emitting diode display (OLED). The thin film transistor for driving the sub-pixel of LCD is used as a voltage-supplying device, and the switching and driving thin film transistor for driving the sub-pixel of OLED is used as a current-supplying device.
For example, each sub-pixel of an active-matrix OLED includes an organic light-emitting diode, which is comprised of an organic light-emitting layer between the cathode and the anode, and a pixel driving unit, which independently drives the organic light-emitting diode. The pixel driving unit adjusts the current amount supplied to the organic light-emitting diode based on a data signal to thereby control the brightness of the organic light-emitting diode. Accordingly, the pixel driving unit includes a switching thin film transistor, a capacitor, and a driving thin film transistor at least.
At this time, the driving thin film transistor directly controls the current amount supplied to the organic light-emitting diode based on a voltage charged in the capacitor through the switching thin film transistor, whereby it requires the rapid current supply. For the rapid current supply, the related art thin film transistor has an increased plane area to increase a channel width where the source and drain electrodes face. However, the pixel driving unit including the driving thin film transistor is disposed in a non-luminescence area which has no organic light-emitting layer. To increase an aperture ratio, it is preferable to decrease the plane area of the thin film transistor. For the thin film transistor that supplies the current to the organic light-emitting diode, it is thus preferable to have both the rapid current supply and the decreased size.